This invention relates to the creation of patterns on photosensitive optical materials by placing the materials in the interference pattern generated by the intersection of at least two beams of light, preferably ultra violet light.
International Patent Application No. PCT/AU96/00782 filed Dec. 2, 1996 entitled xe2x80x9cRing Interferometer Configuration for Writing Gratingsxe2x80x9d (xe2x80x9cthe PCT Applicationxe2x80x9d) discloses a system for writing gratings in photosensitive optical materials. The disclosed system has significant advantages in the reduction of noise characteristics in gratings.
In FIG. 1 of the drawings attached to this specification, there is illustrated a perspective view of an arrangement 30 constructed in accordance with the principles disclosed in the PCT Application. In the arrangement 30 a narrow UV beam is projected onto a phase mask 32. The phase mask 32 produces at 41 two coherent mode beams 33 and 38. One of the beams 33 is reflected at 34 by mirror 35 before again being reflected at 36 by mirror 37 before projecting upon an optical fibre 40 placed at position 45. The second diffracted beam 38 traverses a counter propagating route (not shown) by reflection from mirror 37 and mirror 35. Both of the diffracted beams are constructed so as to impinge upon area 45, resulting in a xe2x80x9cSagnacxe2x80x9d type of arrangement. The two beams 33 and 38, being coherent, form an interference pattern at the point 45. The optical fibre 40, which exhibits photosensitivity, is thereby provided with a modulated refractive index in accordance with the interference pattern. The modulated refractive index is then utilised to form a Bragg grating at the point 45.
The PCT Application further discloses the step of sweeping the UV beam 31 along the phase mask 32 so as to create an extended Bragg grating structure.
FIG. 2 of the attached drawings illustrates the embodiment of FIG. 1 in schematic form and more clearly illustrates the path followed by the reflected beam 33, 34, 36 and the reflected beam 38, 42 and 43.
The PCT Application also discloses rotation of mirrors 35, 37 so as to xe2x80x9cchirpxe2x80x9d the interference pattern 45 and further discloses moving the optical fibre 40 to a staging area where the maximum interference contrast is obtained. This arrangement provides the advantage that the wavelength of the Bragg grating is thereby tunable and, utilising a single phase mask, the wavelength can be lowered by means of increasing the crossing angle of the writing beams. Of course, altering the crossing angle may cause the overlap region of interference 45 to move away from the optical material 40 but, as disclosed in the PCT Application, the movement can be corrected by moving the fibre to a new location of maximum overlap. This can be achieved by utilising, for example, a small translation stage to mount and move the fibre 40. However, the process of translation of the fibre is extremely complex, requiring the turning off of the laser and the accurate repositioning of the fibre with respect to the interference pattern.
Unfortunately, as noted in the arrangement of the PCT application any movement of the reflecting mirrors results in a corresponding movement of the intersection point of the interfering beams and a change in the angle of intersection of the beams. The change in the angle of intersection will result in a consequential change in the Bragg Wavelength written as the Bragg Wavelength is proportional to the angle of intersection.
Hence, as the mirror angles are changed to, for example, chirp the grating, the point of intersection will move away from or towards the mirrors 35, 37. The fact that the point of intersection of the writing beams is directly related to the angles of each mirror 35, 37 means that it is impossible to vary the Bragg wavelength of the grating without the point of intersection moving. If the fibre is positioned parallel to the phase mask, this orthogonal movement in the beam intersection reduces the effective fringe contrast and apodises the grating in a known but unwanted manner. While this effect can be reduced by aligning the axis of the fibre 80 along the path followed by the intersection of the writing beams, this is only effective if you wish to linearly chirp the grating. Therefore, in the arrangement of FIG. 1 and FIG. 2, the orthogonal movement in the writing beam intersection and the Bragg wavelength are not independently controllable, thereby limiting the amount and type of chirp that can be written into a grating without some degree of unwanted self apodisation.
It has also been found in practice that translation of the UV beam 31 along the phase mask 32 results in a corresponding translation of diffracted beams 33, 38 across the surfaces of mirrors 35, 37. Unless the mirrors 35, 37 are perfectly flat, the path of beams 33, 38 will undergo slight variations in angle and intensity as each beam traverses its mirror surface. This results in the introduction of a xe2x80x9cnoisexe2x80x9d factor which can show up in a grating within fibre 40 as unwanted fluctuations in the frequency response of the grating. In particular, where the grating is a chirped Bragg grating and the fibre 40 is utilised as a dispersion compensator in a telecommunication circuit, the variation from a purely linear response can become significant. This is often evidenced as a significant group delay ripple.
If the fibre 40 is positioned parallel to the phase mask 32, the orthogonal movement of the beam intersection point, as a result of mirror movement, can reduce the effective fringe contrast and apodise the grating in a known but unwanted manner. While this effect may be reduced by aligning the axis of the fibre along the path followed by the intersection of the writing beams, this reduction is generally only effective when a linearly chirped blazed grating is required as the non-perpendicular fibre results in a Blazed grating being produced.
Further, due to the geometry of the writing system 30, it is clear that, as the UV beam is scanned from one end of the phase mask to the other, the distances travelled by the two beams from the phase mask to the point of intersection will not be identical. The difference in path length will vary from approximately minus half the scan length to plus half the scan length. For long grating periods, this path length difference can place high demands on the temporal coherence requirements of the UV source and also can effect the spatial stability of the intersecting beams, and is a further potential source of noise in the written interference pattern.
Additionally, because the fibre 40 must be in a plane either below or above the plane of the UV beam 31 the path length difference between the beams, in combination with the small vertical tilt applied to both mirrors, can cause the intersecting beams to move apart vertically as the UV beam is scanned along the phase mask. This vertical separation can also lead to unwanted self apodisation, loss of grating strength and contrast at the ends of the grating.
Further problems exist with the system 30 when the phase mask is xe2x80x9cditheredxe2x80x9d so as to apodise the grating in a desired manner. As the mask 32 is dithered, the fringe contrast will be reduced. This effect can be used to apodise the grating within fibre 40. Dither control is important if the noise on the apodised grating profile is to be reduced. If the dither amplitude is not exactly right then unwanted fringes may be written in the fully apodised regions of the grating. Unfortunately, chirped gratings used for dispersion compensation can be extremely sensitive to imperfections in the apodisation profile. It is, therefore, important that the fringe pattern be smoothly extinguished at each end of the grating. The present utilisation of the process of dithering the phase mask is thought to perhaps introduce both unwanted phase and aperture noise in the apodisation profile, leading to unwanted noise on the transmission spectrum and ripples in the group delay characteristic.
The present invention provides a number of alternative arrangements which alleviate one or more of the aforementioned disadvantages and include an increase in the independent control over the exposure, fringe contrast and period to better approximate ideal gratings.
In accordance with a first aspect of the present invention there is provided a method for creating a grating structure in a photosensitive material, the method comprising the steps of:
utilising a single coherent beam of light and a beam splitting device to create two coherent working beams of light;
propagating the two working beams around a plurality of reflective elements, each of the beams being reflected by each of the reflective elements, so that the beams interfere at an initial predetermined position;
positioning the photosensitive material at a second predetermined position; and
rotating one or more of the reflective elements to simultaneously independently control the period and position of the interference pattern in accordance with predetermined requirements and so as to produce the grating structure
The beam splitting device preferably comprises a diffraction grating and, in such case, the method preferably comprises simultaneously translating the diffraction grating and the photosensitive material in a direction substantially perpendicular to the single coherent beam so as to create the grating structure in the photosensitive material.
The reflective elements may also be translated relative to the photosensitive material so as to maintain the interference pattern substantially focussed on the photosensitive material.
In the alternative, the method may comprise rotating the reflective elements and simultaneously translating the diffraction grating and the photosensitive material in a direction parallel to the single coherent beam so as to maintain the interference pattern at the predetermined position.
In accordance with another aspect of the present invention there is provided a method for creating a grating structure in a photosensitive material comprising the steps of:
utilising a single coherent beam of light and a diffraction grating to create two coherent working beams of light;
propagating the two working beams around a plurality of reflective elements, each of the beams being reflected by each of the reflective elements, so that the beams interfere at an initial predetermined position;
positioning the photosensitive material at a first position so as to produce an initial portion of the grating structure in a predetermined portion of the photosensitive material;
translating the diffraction grating and the photosensitive material in a direction substantially perpendicular to the single coherent beam so as to produce the grating structure in the photosensitive material; and
simultaneously translating the reflective elements relative to the photosensitive material so as to maintain the interference pattern at a position determined in accordance with requirements for the grating structure.
In accordance with a further aspect of the present invention, there is provided a method of writing a Bragg grating in a photosensitive optical waveguide comprising: utilising a single coherent beam of light and a diffraction grating to create two coherent working beams of light; positioning a plurality of reflective elements on each side of said waveguide; propagating the two working beams around said plurality of reflective elements, each of the beams being reflected by each of the reflective elements, so that the beams interfere at an initial predetermined position; and rotating at least one of the reflective elements on each side of the waveguide to simultaneously independently control the period of the interference pattern and position of the interference pattern in accordance with predetermined requirements so as to produce the grating structure in the photosensitive waveguide.